1. Field of the Invention
The invention generally relates to lasers and in particular to a highly efficient, moderately powered, and comparatively low cost diode-pumped solid-state laser and laser amplifier.
2. Description of the Prior Art
Since their introduction in the 1960's, lasers have been incorporated into a wide range of commercial and defense applications in fields as diverse as communications, information storage, information processing, printing, materials processing, and medicine.
Optical power output and input-to-output power conversion efficiency, as well as output wavelength, are characteristics which often determine the suitability of a laser for a particular application. For example, large, continuous-wave gas ion lasers are employed in applications which require about 10 watts of output power. The disadvantages of such lasers include their large size and inefficient performance. In addition, ion lasers of this sort typically require 10 kilowatts of input power and 4 gallons per minute of cooling water.
Solid-state lasers pumped by individual laser diodes have proven to be particularly useful in low power applications (heretofore up to about 2 watts) because of their compact size and efficiency. In the latest generation of diode-pumped solid-state lasers, the lasing material is pumped by means of monolithic laser diode arrays. A wide range of applications can be satisfied by diode-pumped solid state lasers. Techniques like intra-cavity frequency doubling and frequency mixing can be used to manipulate the output wavelength. They can also be configured to operate as optical amplifiers.
However, solid-state lasers have not been able to satisfy applications demanding higher power in a compact package. For example, Sony has demonstrated that a solid-state laser can read and write data on optical disks (compact or video disks). However, lasers with adequate power are not sufficiently compact and economical to help this technology penetrate the consumer market. Similarly, the low powers presently available from solid-state lasers have been a major factor in the slow pace of developing high data-rate intersatellite communications systems.
The source of the major limitation on solid-state lasers is the efficiency and effectiveness of the pump source. The majority of commercial 10 watt solid-state lasers presently sold are based on flash or arc lamp pumped lasers. In such lasers, a large fraction of the pump light is absorbed by portions of the laser rod which do not participate in the generation of laser output. This unutilized pump energy adds additional heating while reducing efficiency. In addition, the lamps have a limited lifetime (up to 1000 hours) for continuous wave applications.
To increase the overall laser efficiency, many groups have sought to develop lasers where the pump light is confined purely to the laser cavity mode. Over twenty years ago, it was shown that longitudinal pumping of lasers could significantly increase the gain and could be a far more efficient optical process than transverse pumping. As early as 1968, Birnbaum et al. (App. Phys. Lett. 12, 225 (1968)) demonstrated pulsed operation of a ruby laser by imaging the output laser light from an argon ion laser longitudinally into the lowest order transverse mode (TEM.sub.00) defined by the cavity of the ruby laser. This laser was optically efficient both because of the geometrical overlap of the pump beam with the laser mode and because of the strong spectral overlap of the pump and the absorption band of the laser medium.
A more recent laser design was the superluminescent diode end pumped neodymium laser, proposed in 1979 by Kubodera et al. (Applied Optics 18(6), 882 (1979)). A relatively high overall electrical efficiency was projected and later achieved by longitudinally pumping a Nd.sup.3+ crystal with a laser diode.
A more important step in this direction was achieved in 1985 by Sipes, who demonstrated that a diode laser phase coupled array could be effectively imaged into the TEM.sub.00 mode of a small Nd:YAG laser. The phase coupled array had an effective aperture of 16 by 100 microns, while the laser mode had a diameter of 180 microns. (Applied Physics Letters, 47(2), 74 (1985)) Sipes observed an increase in the optical conversion efficiency due to the increased pump power density in the small TEM.sub.00 laser mode. The pump power-density (power/unit volume) was increased by focussing the laser diode solely on the portion of laser material contained within the TEM.sub.00 mode. The additional high efficiency of the laser diode resulted in a significantly improved overall device efficiency of 8%. With a few minor modifications to this technique, overall electrical efficiency was increased to 19% by Fields et al. in a diode laser array pumped Nd.sup.3+ :YVO.sub.4 laser. Similar performance was obtained by imaging broad area single stripe laser diodes of similar dimensions and power. These techniques are disclosed in the following patents: Sipes, U.S. Pat. No. 4,710,940, Baer et al., U.S. Pat. No. 4,701,929, and Byer et al. U.S. Pat. No. 4,739,507.
However, lasers which rely on imaging the output from a single diode-laser element are not able to satisfy applications which require 1 watt or more of output power. By diode laser element, we mean a phase coupled group of closely spaced stripes or a single broad-area stripe. Several groups have attempted to obtain higher pump powers by capturing the output of many diode laser elements with fibers and then grouping the fibers into a bundle so that the resultant output can be imaged on the laser rod. This approach falls short, in part due to the limited diode laser-to-fiber coupling efficiency (typically only 50%) and the complexity of coordinating many components.
A more cost effective approach to increased power has been the use of a diode laser bar to pump the laser. Spectra Diode Labs Model No. 3400 series is a typical example. This bar consists of separate groups of gain-guided phase-coupled GaAlAs laser diode stripes, (i.e., several separate diode laser elements). It has been claimed that a 1 cm bar composed of 10 groups of 20 phase-coupled stripes can yield 10 watts of pump power over a reasonably long life (5000-8000 hours). (Elec. Letter 25, 972 (1989)). Similar bars have been developed by others where each element on the bar is a broad area laser.
However, the problem of concentrating the pump light output from such bars, into the laser cavity, has presented significant obstacles to the effective use of these bars. Three relevant approaches have been pursued. In U.S. Pat. 4,818,062, Scifres et al disclosed a technique to couple a gain guided laser bar 11 into a single waveguide 17 or multiple waveguides 103 made up of optical fibers. This approach is limited by the coupling efficiency of the bar to the fibers.
Baer, in U.S. Pat. No. 4,837,771, modified the resonator and quasi-longitudinally pumped solid-state laser material by imaging the spatially separate emissions from a 10 element diode-laser bar onto the reflection points in a tightly folded resonator. This approach is more efficient than the previously mentioned fiber bundle technique and it more effectively distributes the resultant pump-induced heating. However, the tightly folded resonator has demonstrated only about 40% optical conversion for diode pump light to Nd.sup.3+ laser light out. This is a result of the imperfect match between the diode laser pump bar emitters and the tightly folded laser resonator mode.
It was recently shown by Fan et al. that if one could collimate an array of sources, then the array of collimated sources could be focussed by a single large lens to a single spot. (Optics Letters 14, page 1057 (1989)). A group from General Dynamics attempted such a collimation on a standard Spectra Diode Mod. No. 3400 Bar by using an array of 20 microlenses to match up with the gain-guided bar emitters. Only 50% of the diode laser bar light could be effectively collected at the focal spot. The nonuniform wavefront of each of the gain-guided multi-stripe bar elements prevented satisfactory collimation of the light by a single microlens element, thus limiting the final focusing of the all the light sources.